Stack consumption can be seen on the tooltip over the supply icon. On the lower right corner.

Check the supply generated on the region you currently are on the region tooltip. If your stack has a supply wagon it can also gather supply from adjacent controlled regions, so check their supply too.Multiply this numbers by five. If the result is equal or above the supply consumption number of the stack you should be OK.

Yes, the supply produced on a region should be shared among the stacks in the regipn or adjacent to them with a supply wagon.But un case os lack of supply i dont know how the system prioritize where and who it send the supplies.Regards